1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the present invention is related to optical wavelength multiplexing or add/drop devices with high reflection channel isolation and the method for making the same in compact size.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and perhaps more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements.
Optical add/drop and multiplexer/demultiplexer devices are those elements often used in optical systems and networks. For example, an exchanging of data signals involves the exchanging of matching wavelengths from two different sources within an optical network. In other words, an add/drop device can be advantageously used for the multi-channel signal for dropping a wavelength while simultaneously adding a channel with a matching wavelength at the same network node. Likewise, for transmission through a single fiber, a plurality of channel signals are combined via a multiplexer to be a multiplexed signal that is eventually separated or demultiplexed via a demultiplexer.
A typical prior art free-space WDM Demux, as disclosed in U.S. Pat. No. 5,583,683, is duplicated in FIG. 1. In the device 100, a multiple wavelength light traveling in a fiber is separated into multiple narrow spectral bands directed respectively to individual channels. At each of the multiple ports, a dielectric thin film filter transmits a specified wavelength sub-band signal of the multiple-wavelength light collimated and passed by a channel port, and reflects the other wavelengths signals. The remaining multiple wavelengths signals continue to a next channel port, where the in-band signal is transmitted out and the other signals are reflected and propagate along the main path. After multiple bounces, multiple channel signals are respectively demultiplexed.
An OADM (Optical Add/Drop Multiplexer) device works in a slightly different manner compared with a Mux/Demux device. FIG. 2 shows a typical prior art free-space WDM OADM, duplicated from U.S. Pat. No. 6,198,857. In the OADM device 200, at the drop channel port, a thin film filter splits the multiple-wavelength signal according to its spectral characteristics. At the add channel port, the filter does not drop any signals, only reflects all the main-stream signals from the previous channel, meanwhile, inserts a new band signal to the path.
All these free-space devices have the advantages of low loss, reliability and compactness. Compared with conventional three-port cascading modules, the dimension of the free-space devices is significantly reduced as fiber routing in three-port modules is replaced with collimated beams in the free-space devices, thus the routing overhead is skipped. For a fiber, the bend radius is not allowed to be too small. For example, for a widely used SMF-28e fiber, the minimum bending radius is about 30 mm. When being routed, the fiber roll wastes a specified space, 60 mm in diameter for SMF-28e. Without the fiber routing, a free-space WDM device box can be even smaller than a square of 30 mm.
As shown in FIG. 1 and FIG. 2, the fiber input/output ports are from both sides of a package (e.g., a mechanical box). In the process of handling the fiber, due to the minimum radius limitation, the space waste is doubled as shown FIG. 3A. One of the features in the present invention is to overcome such double-sided ports by one-sided fiber fan-out as shown in FIG. 3B with the same routing criteria because routing with a one-sided device saves a half of routing space overhead. In one embodiment of the present invention, as will be detailed below, the I/O ports are on one side of the box, even routing may be eliminated as shown in FIG. 3C.